Ignition Timing for First-Generation GM V-8 Engines

[Swearbody]

Ive concluded with help of a friend that my mechanical advance is bad as i have the distributor hooked to manifold vac at the moment and theres no change in running except maybe a very slight bit better. Im actually contemplating one of those complete skip white distributors and swap the whole shebang.

Mechanical advance has nothing to do with vacuum. It works off the springs and centrifugal force of the weights and rpm.

 

[Vbb199]

I'm having some issues getting my truck to start. It'll crank and back fire (LOUD!!!)

One place I read to find TDC was to remove the driver's side valve cover. When the 1st rocker goes down then up, second rocker goes down then up it should be at TDC; but this would mean both valves are closed.
Am I reading one of them wrong?

This reminds me of a guy we had on the forum that installed his distributor 180° out, and somehow managed to get it to fire, which in turn caused both of his mufflers to explode (he posted pics)

We had many lolz that day.

 

[rich weyand]

Ported vacuum, or timed vacuum, is emissions control nonsense and has nothing to do with how gasoline actually burns. At idle, you'll be dumping burning gas out your exhaust valves so the AIR pump can add air and burn up emissions in the exhaust manifold. No AIR pump? Then why retard the idle ignition?

I remember when they started that ported vacuum nonsense in 1968. First thing you did was rip off the AIR pump and swap the distributor vacuum advance back to manifold vacuum where it belongs.

Just follow carburetor manufacturers instructions as below.

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[Ricko1966]

technically you'd want say a transmission modulator on the drivers side port, and your vacuum advance for distributor on the passenger side port.

No that is not correct. Ported vacuum( passenger side port on eddy 1406 )wasn't even thought of until emissions equipment was thought of. Before emissions standards cars were connected to manifold vacuum( drivers side on eddy 1406.) Ported and manifold vacuum are the same except for at idle. Another thing a lot people do not realize is if you set your idle speed using the throttle stop screw, instead of the idle adjustment screws, is many times you uncover the transition ports in the carburetor which screws up your fuel delivery at low end.
Not saying you can't tweak it a little but there's a fine line between a little and too much.

Damnit I wrote this before reading the post above mine.

Rusty, you didn't ruffle my feathers, I just didn't know if you knew. How people do you think will get tickets this week setting their timing?

 

[QBuff02]

No that is not correct. Ported vacuum( passenger side port on eddy 1406 )wasn't even thought of until emissions equipment was thought of. Before emissions standards cars were connected to manifold vacuum( drivers side on eddy 1406.) Ported and manifold vacuum are the same except for at idle. Another thing a lot people do not realize is if you set your idle speed using the throttle stop screw, instead of the idle adjustment screws, is many times you uncover the transition ports in the carburetor which screws up your fuel delivery at low end.
Not saying you can't tweak it a little but there's a fine line between a little and too much.

Damnit I wrote this before reading the post above mine.

Rusty, you didn't ruffle my feathers, I just didn't know if you knew. How people do you think will get tickets this week setting their timing?

It's not totally incorrect, just not exactly optimum. If you only have two ports and an automatic transmission, you need manifold vacuum to operate the trans vacuum modulator correctly, so for the quick and dirty you could hook your distributor to the ported and trans modulator (it should already be) to the manifold port. It will more than likely require a timing adjustment for sure. best case would be to get a T fitting and tie them both in if you don't have another location for full manifold vacuum. Or just don't run junk edelbrock carbs to begin with. Lol I mean they are good in some applications but I won't have one on any engine that's modified much over stock. I agree with you though that too many people adjust the idle screws as a crutch to make their engine idle correctly and it only compounds the problem further. And then they wonder why! I set the butterflies and transition before installing the carb and then use a vacuum gauge to tune the carb once it's installed, that's the best way to do it I think. I run an 850 Demon carb on my Big Block and after about 3 trips around it with a vacuum gauge the throttle response is about as crisp as fuel injection. And that's part of the other problem, a lot of people don't want to invest the time into it.

 

[Raider L]

@rich weyand,

In addition to how I have my dist. set as I talked about above, I failed to mention the next thing I go to is to set my carb. idle jets, according to where the rpm is after the timing is set. I have gauges installed my engine block just for timing. I have a small tach over on the inner wheel well so I know what the engine is doing. Then on a bracket installed on the front of the engine I have a vacuum gauge, and a front oil pressure gauge. The vacuum gauge is used so I can start turning the idle jets setting them for max intake vacuum on each side of the carb. I can tell what is going on after I have the timing light looking down at the harmonizing wheel when I'm at max advance as to what the springs are doing all the way to 3,000 rpm. I can do ignition and carb. settings on the side of the road. I hardly ever have to change the idle screw. Usually setting the idle jets does all I need to do with no waste of fuel.

I don't know why my engine won't run as well as it runs now with a vacuum advance on it, but I took that thing off long ago and haven't looked back. Here recently I took the cap off to change it, I could tell it needed it, and also changed out the springs for new ones to renew anything there.

And also let me mention this, the weight of the counter weights makes a big difference when you are using the springs and weights for all the advance you are going to have. The counter weights come in grams. Some allow the "all in" to come in to fast no mater what stiffness spring you've got on there. I know I've tried them all. And everyone makes them, Moroso, Mr. Gasket, Accel, MSD, Mallory I think, etc., etc. I'm using the set I got with my MSD distributor, I don't remember what they weigh but I know they are a lot heavier than the Mr. Gasket's. The counter weights are matched with the springs that come with them. Each mauf. thinks they know what works with their counter weights. I know because I've mixed them up before to see what would happen, you are better off staying with the one's that come with the counter weights. I have papers and charts for particular cam, engine hp/torq cylinder bore, rod length (piston speed), etc. to get everything set like it should be. Although it still needs a bit of tweaking and just a tad of best guess to get it just right. It's fun, interesting, and a challenge to do this and have what you've done work.